1. Field of the Invention
The present invention relates to a heat resistant polyimide film and its manufacturing method and, more particularly, to a polyimide film whose adhesion performance is improved by sizably reducing its residual content of volatiles from that of the conventional counterpart, and further by, besides reducing the residual content of volatiles, controlling the oxygen/carbon ratio of the surface layer, and the manufacturing method thereof.
2. Description of the Prior Art
A polyimide film is known for its outstanding properties such as high heat resistance, cold resistance, resistance to chemicals, electrical insulation performance and mechanical strength, and is being widely used as electric insulation film, heat insulation film and base film for flexible printed circuit board (PCB). In its principal field of application e.g. as material of PCB or electric insulation film, it is often the case that the film is bonded with copper foil by the use of a proper adhesive, used as material of prepreg which is prepared by coating with an adhesive or conjugating with a fluorine resin, hence a great importance is attached to its adhesion performance.
Hitherto, as adhesion performance-imparting techniques for high-polymer films have been known such treating methods as flame treatment, corona discharge treatment, ultraviolet ray treatment, alkali treatment, primer treatment and sandblast treatment. For treatment of polyimide film, too, common methods which can satisfy the purposes of a heat resistant film such as sandblast or alkali treatment have been used.
All these methods are, however, intended for improving its adhesion performance by after-treatment of commercialized films. Thus, any of these methods is not applicable to any film in the process of its manufacture or film forming for improving its adhesion performance, hence there has been sometimes problematic about varied adhesion performance of film before after-treatment as well as stability of after-treating method, homogeneity of film to be treated, and it has been basically difficult to supply stably films of improved adhesion performance. Moreover, commercialization of such a process requires a large amount of investment, this inevitably resulting in a rise of manufacturing cost. The same is the case with a film made by conjugation with e.g. fluorine resin, and it has been difficult to stably realize a film of high peeling strength by any of the conventional techniques.